1. Field of the invention
The invention relates to a liquid crystal cell--also referred to below as display cell--having a ferro-electric, chiral smectic liquid crystal layer.
Known liquid crystal cells have a nematic liquid crystal configuration and, in accordance with this configuration, are referred to as TN or STN cells. Here, TN is Twisted Nematic and STN is Super Twisted Nematic. Such cells are adequate for many applications. However, their image quality is dependent to a very great extent on the user's angle of view. Furthermore, TN and STN cells require a continuously applied signal voltage if they are to be used for displaying or storing information. This means that the cells are not made for storage and/or representation of data without an energy source. An example of such an application would be a bank or credit card which, without an integrated battery, can store the account balance transferred by a write and read device to the card.
Liquid crystal cells having a ferroelectric, chiral smectic liquid crystal layer belong to a different cell type. Cells of this type have in fact a refractive liquid crystal layer which is also referred to below as an S.sub.c * layer and optionally forms a helical configuration and which can be influenced or deformed by the action of an electric field so that its optical anisotropy changes.
Known ferroelectric liquid crystal cells have a pair of transparent plates which together enclose the S.sub.c * layer and are each provided with a surface structure orienting the molecules of the S.sub.c * layer, provided with electrodes for generating an electric field and each provided with a polarizer. Here, the surface structures facing the S.sub.c * layer have an orienting effect on the adjacent liquid crystal molecules, as will be described in more detail below. The smectic layers or planes are perpendicular to the plates in ferroelectric liquid crystal cells. In the literature, such a configuration is occasionally also referred to as the so-called bookshelf structure.
The liquid crystals which can be used for ferroelectric display cells consist of a mixture of different chemical compounds. These include so-called dopants, which impart the chiral character to the mixture and are responsible for the ferroelectricity. A chiral smectic liquid crystal mixture is distinguished in particular by the fact that the molecules belonging to a smectic layer and arranged essentially parallel to one another are arranged not perpendicular to the smectic plane but at a smectic tilt angle .theta. relative to the plane normal. The chirality of the S.sub.c * layer results in the axes of the liquid crystal molecules being rotated relative to one another from layer to layer, resulting in the formation of a helix having the pitch p. If such a helix structure is undesired, it must be suppressed through the effect of the border or modification of the bookshelf structure.
In the rest state, i.e. without an applied electric field, the ferroelectric display cell exhibits a certain light transmittance. If a voltage is applied to the electrodes, torques act on the individual smectic layers. These torques result in a reorientation of the molecules and hence a reorientation of the individual smectic layers, leading to rotation of the optical axis. The rotation of the optical axis as a function of the applied voltage can be determined in practice by measuring the switching angle .alpha..
However, chiral smectic liquid crystal mixtures also have other properties. Thus, they exhibit spontaneous polarization P.sub.s, i.e. an inherent, spontaneous orientation of the molecular dipole moments. This means that an electric field applied to the display cells shows a strong interaction with this spontaneous polarization, permitting a substantial reduction of the switching times known for TN and STN cells. A further property of tilted smectic phases is evident in the case of the bookshelf structure, in which the smectic planes --as already mentioned--are perpendicular to the display surface. If the glass surface is treated here in such a way that molecules of the smectic layers are oriented parallel to the border, there are evidently two possibilities for tilting the molecules relative to the plane normals; namely either forward or backward. The two positions make the same angle with the normals to the S.sub.c planes and are thus compatible with the S.sub.c structure. These two configurations or states are the basis of the bistability of the S.sub.c liquid crystals, i.e. the basis for the formation of bistable ferroelectric liquid crystal cells.
Ferroelectric bistable liquid crystal cells do not have the abovementioned disadvantages of nematic displays. Thus, they have little dependency on the viewing angle and can be disconnected from the energy source as soon as the information has been recorded.
In bistable liquid crystal cells, the ferroelectricity of the S.sub.c * layer can be switched back and forth between the two configurations or states. At the same time, the spontaneously oriented dipole moments of the chiral dopants produce spontaneous polarization P.sub.s, which in turn reacts strongly to an applied electric field. The dopants are thus responsible for the short switching times of bistable cells. It is known that the formation of a helix in a form as occurs, for example, in a DHF cell (here, DHF is Deformed Helix Ferroelectric) is undesired for a bistable liquid crystal configuration. Thus, a limit is imposed on the magnitude of the spontaneous polarization P.sub.s. In fact, the chiral molecules contained in the mixture not only produce spontaneous polarization but also promote the tendency to rotation of the S.sub.c structure, which is disadvantageous for a bistable liquid crystal configuration.
In those bistable display cells which are based on the so-called Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) effect of Lagerwall and Clark (Appl. Phys. Lett. 38, 899, 1980), the doping must therefore be chosen to be so low that the interaction of the molecules with the surface is stronger than the effect of the rotating power of the dopants, suppressing or inhibiting the formation of the helix. In practice, this means that the pitch of the helix must be substantially greater than the layer thickness d of the liquid crystal layer.
The relatively weak stabilization of the structure by surface forces and the partial compensation of these forces by the rotating power of the dopants means that bistable switching can be effected with little energy, but the state thus achieved is very labile. Thus, even relatively small electrical or mechanical interferences can "flip" the state, i.e. cause switching back and forth between the two bistable configurations. SSFLC display cells are therefore not very suitable for safe long-term storage.
2. Discussion of the Prior Art
The so-called Short-pitch Bistable Ferroelectric display cell (SBFLC) offers a solution to this problem. In this cell, the liquid crystal layer is shaped by electric pulses so that a zig-zag modulation of the smectic layers is produced, suppressing the formation of a helix. Consequently, this cell reacts to a substantially smaller extent to vibrations than the known SSFCL cells. A display cell of this type is disclosed, for example, in EP-A 0 405 346.
Both the SSFLC and the SBFLC display cells have a further serious disadvantage. In the cells, the information is burnt in. If in fact the cell remains in the same state for a long time, ionic impurities in the form of charge clouds are produced close to the contacts--due to the electric field of the spontaneous polarization. These three-dimensional charges and possibly additional influenced surface charges change the corresponding working point of the cell for further switching. Ghost images or even loss of bistability are a consequence. It has now been found that these three-dimensional charge effects can be reduced or made less critical if conductive orientation layers are used and the poorly conducting or even insulating layers usual in TN and STN display cells are dispensed with. However, the liquid crystal then has a low-resistance connection to the two contacts, dramatically increasing the danger of short-circuits by dust particles, etc.
Another possibility for influencing the helix or director structure of the S.sub.c * layer is the admixture of polymerizable molecules. The idea here is to stabilize or freeze the director structure in an optimal configuration by a polymerization process in the forming or production of the liquid crystal cell. This is achieved by mixing with the liquid crystal mixture a small percentage of polymerizable molecules which in turn have at least one reaction center and initiate a polymerization reaction on exposure to UV light. This polymerization results in the formation of long molecular filaments which wind up into a tight coil and optionally also undergo crosslinking (which results in a network) and thus freezes the director structure present at the beginning of the polymerization reaction. This process has already been described in U.S. Pat. No. 5,434,685. However, the technique described in this prior publication is limited to SSFCL display cells, i.e. to liquid crystal mixtures having a long pitch p. In the liquid crystal cells disclosed in U.S. Pat. No. 5,434,685, moreover, one of the two stable states--namely that in which the liquid crystal mixture is polymerized--will always be more stable than the other, which, during operation of a cell formed in this manner, results in an undesired asymmetry of the required actuating voltages and may even result in the loss of bistability. In the liquid crystal cells disclosed in U.S. Pat. No. 5,434,685, the polymerization can finally also trigger a phase separation, i.e. the liquid crystal is in this case precipitated in the form of small droplets. The display cells described by J. W. Doane, D. K. Yang and L. C. Chien in Conf. Reports IDRC, SID, page 175 (1991) are examples of this.